Electronic ballast is needed to drive a hot cathode fluorescent lamp (HCFL). The electronic ballast needs to provide both preheating power for the filaments and striking voltage to ignite the lamp. After the lamp is ignited, the electronic ballast should regulate the lamp current and continue to provide heating power, though at less level, for the filaments. For the conservation of energy, it is preferred an electronic ballast is capable of dimming control. When HCFL is operated at various dimming conditions, the heating power to the filaments should be adjusted accordingly to ensure a normal life of filaments. Accordingly, the present invention provides a control circuit that provides both preheating power to the filaments, and variable dimming control of the lamp.
Accordingly, the present invention provides an electronic ballast system comprising a variable voltage source generating a first signal indicative of a desired dim value for a hot cathode fluorescent lamp, and a second signal indicative of the average power of said variable voltage source. A ballast controller is provided that includes lamp filament current control circuitry comprising preheat filament current control circuitry generating a preheat filament current to the filaments of the lamp for a predetermined time period, and steady state filament current control circuitry generating a steady state filament heating current in reverse proportion to the desired dim value during times after said predetermined period of time. The controller also includes dimming circuitry comprising a burst PWM (pulse width modulated) signal generator receiving said first signal and generating a PWM dimming signal proportional to a desired dim value, current feedback circuitry receiving a signal indicative of the current supplied to said lamp and comparing said signal indicative of the current supplied to said lamp and said PWM dimming signal to generate a variable power control signal; and inverter circuitry receiving said variable power control signal and generating an AC signal proportional to said power control signal by inverting said second signal. The ballast system further includes output circuitry coupled to said inverter circuitry comprising a resonant tank circuit receiving said AC signal to deliver striking and steady state sinusoidal power to said lamp.
In another embodiment, the present invention provides an electronic ballast system comprising a variable voltage source generating a first signal indicative of a desired dim value for a hot cathode fluorescent lamp, and a second signal indicative of the average power of said variable voltage source. A ballast controller is provided that includes lamp filament current control circuitry comprising preheat filament current control circuitry generating a preheat filament current to the filaments of said lamp for a predetermined time period and a steady state filament current control circuit generating a steady state filament heating current during times after said predetermined period of time; dimming circuitry to vary the power delivered to said lamp as a function of the value of said first signal; and a full bridge inverter circuit generating an AC signal from said second signal based on said dimming circuitry. The ballast also includes output circuitry coupled to the output of said full bridge inverter comprising a resonant tank circuit receiving said AC signal and generating a sinusoidal signal to deliver striking and steady state power to said lamp.
It will be appreciated by those skilled in the art that although the following detailed description will proceed with reference being made to exemplary embodiments and methods of use, the present invention is not intended to be limited to these exemplary embodiments and methods of use. Rather, the present invention is of broad scope and is intended to be limited only as set forth in the accompanying claims.
Other features of the present invention will become apparent to those skilled in the art as the following detailed description proceeds, and upon reference to the drawings, wherein like numerals depict like parts, and wherein: